Effect of Dissolved Gas to NPSH available.
Effect of Dissolved Gas to NPSH available.
(OP)
Effect of Dissolved Gas to NPSH available.
Dear all,
I would like to know what is the effect of dissolved gas to NPSH available calculation. My liquid is shipping oil. This shipping oil have undergo gas-liquid separation and oil-water separation. However, my HYSYS simulation shows that in the shipping oil stream, there is some CO2, methane and ethane which I assume are dissolved into the oil stream (0.0025 mole fraction). This dissolved gas increase the vapor pressure calculated by HYSYS.
It seem that at a pressure above the oil vapor pressure, this dissolve gas start to break from liquid.
1. My question is do the gas bubble in the suction of pump that source from breaking of dissolved gas has the same effect with vapor bubble generate from liquid it self due to suction pressure is lower than liquid vapor pressure? Is there any reference?
2. If yes do it means that in system that liquid has contained dissolved gases, the suction pressure should keep quite high in order to prevent this dissolved gases to break and harm the impeller?
Thanks for all attention
Dear all,
I would like to know what is the effect of dissolved gas to NPSH available calculation. My liquid is shipping oil. This shipping oil have undergo gas-liquid separation and oil-water separation. However, my HYSYS simulation shows that in the shipping oil stream, there is some CO2, methane and ethane which I assume are dissolved into the oil stream (0.0025 mole fraction). This dissolved gas increase the vapor pressure calculated by HYSYS.
It seem that at a pressure above the oil vapor pressure, this dissolve gas start to break from liquid.
1. My question is do the gas bubble in the suction of pump that source from breaking of dissolved gas has the same effect with vapor bubble generate from liquid it self due to suction pressure is lower than liquid vapor pressure? Is there any reference?
2. If yes do it means that in system that liquid has contained dissolved gases, the suction pressure should keep quite high in order to prevent this dissolved gases to break and harm the impeller?
Thanks for all attention
-rayz-





RE: Effect of Dissolved Gas to NPSH available.
If the pressure drops and you get two phases then you have bubbles that may collaps after entering the pump - causing cavitation.
On the other hand - in real life you may have some gasses entrained in the liquid - e.g. N2 or even CO2 and light HC such as methan etc -
But HYSYS cant really handle this (at least not the SS verion) and will calculate that they seperate out as a second phase - and will thus calculate composition and volume of each phase.
Best regards
Morten
RE: Effect of Dissolved Gas to NPSH available.
Thanks for quick response, I agree that HYSYS are not reliable on entrainned gases.
However my major question has not answered yet. In real life if there is any dissolve gas (entrained gas) in my liquid, did I will have cavitation caused by bubble of gases that is break when pressure decrease in impeller eye then collapsing when pressure increase back.
-rayz-
RE: Effect of Dissolved Gas to NPSH available.
If you look at the phase diagram of the entrained gases that you are pumping you will see if the pump causes them to cross a phase boundary.
David
RE: Effect of Dissolved Gas to NPSH available.
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RE: Effect of Dissolved Gas to NPSH available.
RE: Effect of Dissolved Gas to NPSH available.
Here is what you know, whats in the tank is at an equalibrium temperature and pressure (I'll clarify this later on). As the liquid flows down to the bottom of the tank the fluid gains pressure from the head and losses pressure due to frictional loss. So just make those calculations. These all work the same as when doing the calcs with pure water.
The problems come in with calculating head gain from the density. Do we really know the density? The best bet is to assume the lightest density that could be in the system and use that for calculating head gain.
Back to equalibrium. There is a time dependant variable on equalibrium. I've had a pumping system taking propane (which is actually 95% C3 and the remaining other C1 to C5 componds) from a tank at its bubble point of 195 psig, and pumped into trucks. When pumping at 250 gpm from a 30,000 gallon tank, the pumpo vapour locks. If I pump at the same reate from 60,000 gallon tanks, the pump doesn't vapour lock. Why? Because in the 30,000 gallon tank the boiling propane does not release the C1 to C3 molecules quick enougn and they do not "vapor out" until they are in the line to the pump. Lesson learned, either slow down the pump of allow for more NPSH.
RE: Effect of Dissolved Gas to NPSH available.
RE: Effect of Dissolved Gas to NPSH available.
Not the same situation exactly, as different application: it was an emergency pump (didn't normally run) which had to meet certain flow requirements, fluid was water, utility ran the NPSH/ sumbmergence calcs and found they were several inches to the negative and made an assumption that the pump could handle 6% voiding with no problem. When they finally got around to asking the vendor (after some prodding), the vendor refused to certify his pump would perform as required. Utility had to make modifications to raise minimum level to ensure pump would run.
Patricia Lougheed
Please see FAQ731-376: Eng-Tips.com Forum Policies: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.
RE: Effect of Dissolved Gas to NPSH available.
1) "Inert gas in liquid mars pump performance", Chemical Engineering Magazine, July 3, 1978
2) Tsai, M.J. "Accounting for Dissolved Gases in Pump Design", Chemical Engineering, July 26, 1982, pp65-69
3) Chen C.C. et al, "Cope with Dissolved Gases in Pump Calculations", Chemical Engineering, October 01, 1993
4) Wood D.W. et al, "Pumping Liquids Loaded with Dissolved Gases", Chemical Engineering, July 01, 1998
5) "Effects of Entrained Air, NPSH Margin, and Suction Piping on Cavitation in Centrifugal Pumps", Pumping Technology, June 1998
Best Regards
RE: Effect of Dissolved Gas to NPSH available.
Having insuficient NPSHa is not the same as having air entrainment, if NPSHr exceeded NPSHa I wouldn't approve it either.
For this application (emergency pump)I would have l would have introduced some air (via control valve)into the inlet to soften the cavitation.
RE: Effect of Dissolved Gas to NPSH available.
They had sufficient NPSH, not sufficient submergence. They were introducing air quite well.
Patricia Lougheed
Please see FAQ731-376: Eng-Tips.com Forum Policies: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.
RE: Effect of Dissolved Gas to NPSH available.
Seems you are talking about a vertical pump of some description - right?
If submergence is not required as part of the NPSHa consideration and air is being taken into the pump, then it becomes a problem of reducing air entrainment due to the vortexing and there are a number steps you can take to overcome this problem.